The present invention concerns the field of transponders and, more particularly, transponders for half duplex communication.
There exist conventionally a large number of portable devices for allowing identification from a distance of an unknown object. Typically, data are transferred in the form of radioelectric signals, between a base station and a portable device which is usually made using a transponder.
FIG. 1 shows a conventional transponder 1 which includes an antenna 3 for receiving and transmitting data in the form of radioelectric signals, processing means 5 connected to antenna 3 for processing the received data and the data to be transmitted, supply means 7 for providing a supply voltage Vdd to the different components of transponder 1, in particular to processing means 5.
Half duplex communication between the base station and transponder 1 includes two operating phases: a reception phase and a transmission phase.
During the reception phase, the base station transmits a radioelectric signal X1, antenna 3 receives it and converts it into an electric voltage U.sub.1. The base station can also supply the electric energy necessary for the electric supply of transponder 1, by transmitting signal X1 at a sufficiently high electric power. In this particular case, transponder 1 operates like a &lt;&lt;passive &gt;&gt; component, and supply means 7 include a storage capacitor intended for storing said electric energy.
During the transmission phase, an electric voltage U.sub.2 is present across the terminals of processing means 5, and is supplied to antenna 3 which converts it into a radioelectric signal X2. In the particular case in which transponder 1 is passive, capacitor 7 which has stored electric energy during the reception phase, supplies the electric power necessary for the operation of processing means 5 during the transmission phase.
One problem encountered during such communication lies in the optimization of the electric energy transferred between antenna 3 and processing means 5.
A conventional solution to this problem consists in connecting a capacitor 8 in parallel to antenna 3, so as to form a circuit of the parallel LC type, as is shown in FIG. 1. This circuit can supply voltage U.sub.1 at a maximum amplitude, when the frequency of such voltage is equal to the resonance frequency fo of such circuit. It is then said that the LC circuit is voltage resonant. It will be recalled that resonance frequency fo of an LC circuit is defined as follows: ##EQU1##
By way of illustration, FIG. 2 shows a wave shape 9 illustrating the change in voltage U.sub.1 as a function of its frequency f, in the case where the circuit is of the parallel LC type. Thus, when frequency f of this voltage is substantially equal to said resonance frequency fo, the amplitude of voltage U.sub.1 is maximum, the reference U.sub.1max designating said maximum voltage value.
Consequently, recalling that the mean power absorbed by processing means 5 is directly proportional to the square of voltage value U.sub.1max, this electric power is then optimum during the reception phase of a communication of the aforementioned type, supposing that signal X1 is periodic at frequency fo. In other words, the transfer of electric energy between antenna 3 and processing means 5 is optimum during the reception phase.
One drawback of the aforementioned conventional solution lies in the fact that, during the transmission phase, the parallel LC circuit does not allow the transfer of energy between processing means 5 and antenna 3 to be optimized. Indeed, during this phase, in the event that the circuit connected to the terminals of processing means 5 is of the parallel LC type, said means provide a current I2, and the voltage U2 present across the terminals of said means is limited by its input characteristics. There results a limitation of the energy present in antenna 3, as well as the magnetic field generated by such antenna during the transmission phase.
The Applicant of the present invention has thus observed that the conventional transponders such as that described in relation to FIG. 1, do not satisfactorily answer the aforementioned problem, since the exchange of electric energy during a half duplex communication is not optimum, as was described in detail hereinbefore.